Low-alloy, high-hardenability steel with high toughness at high hardness levels



Patented Feb. 19, 1952 LOW-ALLOY;HIGHJIKRDENABILITY STEEL WITH HIGH IQOUGHNESS AT HIGH HARD NEss- LEVELS John M. Hodge and Richard 1). Manning, Fittehurgh, Pal, assignors to United States Steel Company, a corporation 6f New Jersey,

Nat-Drawing; Application April 6, 1951, Serial No. 219,771

2' claims. (01. 05-128) This lnvention relates to an improved low-alloy, high-strength steel characterized by high hardenability and by high impact strength after heat treatment to develop high strength or hardness.

Tough, high-strength, i. e., over 220,000 p. s. i. tensile strength, steels find many applications in aircraft, railroad, and automotive equipment where high-tensile strength combined with high iinpac't strength often allows, through the use'of somewhat lighter sections, a weight reduction with no sacrifice in safety. Some typical examples in which high toughness-at high strength is" required include landing-gear struts for large aircraft, rock-drill bitsijfor pneumatic tools, and perforating guns for oil-we'll equipment. it has been the practice in the past to use steels of rather high-alley ccntents; particularly highi'iickel contents, for such appucaucris. Steels for these applications must have sufficient hardenabilityto harden relat vely large sectionsth'r'cu hout! Despite their high-alloy contents, many of these steels are quite" brittle when heat-treated to high-strength levels. In fact, John Thompsoil, has 'shown, in an article amieanng in Irori Age for 8, I945, that the impact strengths of most of the alloy steels used in aircraft applications vary inversely with the hardness or tensile strength and that at hardnesses of 48 to 50 Rockwell-C or tensile strengths of 240,000 to 260,000 p.-s1- lL-these steels will have low; room-temperature impact strengths, that is, on the order of 5 to' lft. -lbs.- Thompson further states that any low-alloy steel heat-treated wa specified hardness or tensi1e 'strength will have about the same impact strengtnasan cther low-alloy steel-heat-treated to that-hardness.

In general, steels of suflicient hardenability to harden large sections throughout, contain relatively large amounts of alloying elements, particularly large amounts of nickel, one of the most expensive alloyingelements. For example, a total alloy content of 6.35% is characteristic of the frequently used 49% nickel-1.25% chromium steel. Likewise, AISI type4340 steel has also been widely used for the high-strength applications to which the present invention is directed, and, although the total alloy content of the mean composition of the AISI 4340 specification range is only 3.85%, the nickel content of this steel may vary from 1.65 to 2.00%.

We have discovered on the contrary that not only can the high hardenability desired be attained without the foregoing loss of impact strength but moreover it can be obtained with than 4% total alloy content along with less than 1 of nickel. Our improved steel containing less than 1% nickel has im act strengths equal to or better than expensive 4% nickel steels not only at room temperature but at temperatures as low as -200 F. V

The chemical compositions of two steels typical of the present invention are given in Table'I and the deep hardening characteristics of the two steels are indicated in Table II. For comparison, there is also included in Table II the lower limit of the hardenability' band for A181 4340 steel.

TABLE I Chemical composition-mercent Steel 0 Mn Si ,Nl Cr Mo B- TAEIJE I'I" Enmuemiv hai'dena-bil'z'ty" test results llardrf'ss liockwell 0" Scale Distance from Quenched- EmIiJ1 11111 S1xtee11ths of AISI 4340 Lower an SteelA SteelB LimitofHai-denability Band 48.5 ;.5 52.5 4s 54 52.5 48 53 52.5 47.5 53 52 47v .3 -51 47 5a 50 45 52.5 40

Nora: The difierences in the carbon contents of the steels account for the difie'r'ei'ices' ill hardnes's'levels.

The heat treatment of thesteel of this in-'- 'vention involves conventional practices. After being austenitized at a temperature high enough and for a time long enough to effect adequate carbide solution, the steel is cooled in oil or water at a rate sufficiently rapid to avoid transformation at high temperatures. In this way analmost wholly martensitic microstiuct'ure is obtained. This quenching treatment is followed by a tempering treatment preferably in the range 400 to 500 F., the exact temperature depending on the carbon content and on the desired strength level. Tensile-strength levels between about 220,000 and 270,000 p. s. i. may be obtained from this steel by judicious selection of carbon content and of tempering temperature. After heat treatment, V-notch Charpybar impact strengths of about 25 to 28 ft.-1bs,

TABLE III Heat treatment Oil-quenched from 1600 F. and tempered at Mechanical properties Hardness, Rockwell C" 46 Yield strength, p. s. i. (0.2% ofiset) 185,900 Tensile strength, p. s. i. 226,300 Elongation in 1 inch, percent 14.0 Reduction in area, percent 57.9 V-notch, Charpy-bar impact values, ft.-lbs:

Tested at room temperature 28 Tested at 100 F 24 Tested at 200 F. 20 Fatigue strength, p. s. i. 118,000

Table IV shows the properties of the 0.35% carbon steel B, and also a direct comparison of its properties with the properties of a more expensive, high-nickel steel of the following composition.

Per cent Carbon .34 Manganese .46 Silicon .24 Nickel 4.38 Chromium 1.27

It is readily apparent that this steel contains more than 4% total alloying elements and also more than 1% nickel.

TABLE IV Heat treatment Both steels were oil-quenched from 1600 F. and tempered at 400 F.

Mechanical properties High- SteelB Nickel Steel Hardness, Rockwell "C 51 50. 5 Yield Strength, p. s. i. (0.2% ofiset) 210, 500 210,300 Tensile Strength, p. s. i 267, 600 261, 700 Elongation in 2 inches, per cent. 13.0 13. 5 Reduction in Area, per cent 43. 6 43. 4 V-Notch, Charpy-bar impact values, f bs.:

Tested at room temperature 26 24 Tested at -100 F. 21 20 Tested at 200 F 19 19 The desirable combination of properties hereinbefore described can be obtained within the following composition ranges:

The remainder of the steel is substantially all iron except for the normal amounts of residual impurities consistent with killed-steel practice.

While we have described two specific embodiments of our invention, it will be understood that these embodiments are merely for the purpose of illustration and description and that various other forms may be devised within the scope of our invention, as defined in the appended claims.

We claim:

1. A high-hardenability steel characterized by less than 4% total alloy content and less than 1% nickel and tensile strengths in excess of 220,000 p. s. i. and a V-Notch, Charpy-bar impact value in excess of 20 ft.-lbs. at 100 F. after heat treating by cooling sufliciently rapidly from an austenitizing temperature to obtain a martensitic structure and thereafter tempering within the range of 400 to 500 F., said steel comprising .22 to 87% carbon, .65 to .95% manganese, .55 to .75% silicon, .70 to nickel, .70 to .90% chromium, .40 to .60% molybdenum, .002 to 006% boron, balance iron and residual amounts of other elements.

2. A high-hardenability steel having less than 4% total alloy content together with less than 1% nickel and having a tempered martensitic structure resulting from rapidly quenching it from an austenitizing temperature and thereafter tempering it in the range of 400 to 500 F. characterized by a tensile strength in excess of 220,000 p. s. i. and a V-Notch, Charpy-bar impact value in excess of 20 ft.-lbs. at F., said steel comprising .22 to 37% carbon, .65 to .95% manganese, .55 to .75% silicon, .70 to .95% nickel, .70 to 90% chromium, .40 to .60% molybdenum, .002 to 006% boron, balance iron and residual amounts of other elements.

JOHN M. HODGE. RICHARD D. MANNING.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,478,420 Payson Aug. 9, 1949 2,528,867 Day Nov. 7, 1950 OTHER REFERENCES Transactions of the American Society for Metals, vol. 40, 1948, pages 1102 to 1104, published by the American Society for Metals, Cleveland, Ohio. 

1. A HIGH-HARDENABILITY STEEL CHARACTERIZED BY LESS THAN 4% TOTAL ALLOY CONTENT AND LESS THAN 1% NICKEL AND TENSILE STRENGTHS IN EXCESS OF 220,000 P. S. I. AND A V-NOTCH, CHARPY-BAR IMPACT VALUE IN EXCESS OF 20 FT.-LBS. AT -100* F. AFTER HEAT TREATING BY COOLING SUFFICIENTLY RAPIDLY FROM AN AUSTENITIZING TEMPERATURE TO OBTAIN A MARTENSITIC STRUCTURE AND THEREAFTER TEMPERING WITHIN THE RANGE OF 400 TO 500* F., SAID STEEL COMPRISING .22 TO 37% MOLYBDENUM, MANGANESE, .55 TO .75% SILICON, .70 TO .95% NICKEL, .70 TO .90% CHROMIUM, .40 TO .60% MOLYBDENUM, .002 TO .006% BORON, BALANCE IRON AND RESIDUAL AMOUNTS OF OTHER ELEMENTS. 